# include <iostream>
#include <cmath>
using namespace std;
/* this program is used to calculate the area of triangle*/
int main ()
{double a,b,c,h,s;
cout<<"please enter the length of triangle edge"<<endl;
cin>>a>>b>>c;
if(a+b>c&&b+c>a&&a+c>b)
{h=(a+b+c)/2.0;
s=sqrt(h*(h-a)*(h-b)*(h-c));
cout<<"the aera of the triangle is"<<s<<endl;
}
else cout<<"error"<<endl;
return 0;
}